1. Field of the Invention
This invention relates to an air fuel ratio control apparatus for an internal combustion engine and particularly to an air fuel ratio control apparatus for an internal combustion engine, having first and second oxygen sensors provided at downstream and upstream from a catalytic converter of the internal combustion engine respectively.
2. Description of the Prior Art
An air fuel ratio control apparatus for an internal combustion engine is known which controls an air fuel ratio of an internal combustion engine around a theoretical air fuel ratio in accordance with an output signal of an oxygen sensor (O.sub.2 sensor) provided upstream from a catalytic converter to increase an purification ratio of the exhaust gas.
Moreover, an air fuel ratio control apparatus for an internal combustion engine is known which comprises first and second oxygen sensors provided downstream and upstream from a catalytic converter of the internal combustion engine and compensates an output of the first oxygen sensor provided upstream from the catalytic converter in accordance with an output of the second oxygen sensor provided downstream from the catalytic converter to prevent decrease in controlability due to change or dispersion in a characteristic of the first oxygen sensor. For example, a delay time of the air fuel ratio control by the first oxygen sensor is controlled in accordance with the output of the second oxygen sensor. This is described in Japanese patent application provisional publication No. 61-286550 (corresponding patent application, U.S. Pat. No. 4,739,614).
FIG. 3A is a cross-sectional view of the prior art oxygen sensor mentioned above. FIG. 3B is an enlarged view of a portion A shown in FIG. 3A. As shown in FIG. 3A, each of two oxygen sensors used in the air fuel ratio control apparatus mentioned above comprises an well-known solid electrolyte 31 formed in a tube with one open end and one close end, made of zirconia (ZrO.sub.2) to which yttrium oxide (Y.sub.2 O.sub.3) or the like is add, electrodes 32 and 33 made of platinum or the like provided at inner surface and outer surfaces of the tube of the solid electrolyte 31, and a protective layer 34 for protecting the oxygen sensor itself. This oxygen sensor does not have any function for oxidizing or reducing specific substances in an unbalanced exhaust gas, for example, carbon monoxide (CO), hydrogen (H.sub.2), nitric oxide (NOx), or the like.
Therefore, the presence of the specific substances causes the characteristic of take first oxygen sensor provided upstream from the catalytic converter to deviate from the characteristic in the condition that these substances would be absent, to the lean or rich condition. More specifically, for example, if a large of amount of CO or H.sub.2 are included in an exhaust gas, the characteristic of the first oxygen sensor deviates from that in the normal condition to the condition of lean in oxygen density because an interval is required for sufficient oxidizing reaction on the surface of the electrode 33. Therefore, such deviation prevents surer control of the air fuel ratio of the internal combustion engine to a theoretical air fuel ratio.
The characteristic of the second oxygen sensor provided downstream from the catalytic converter does not largely deviate to on the condition of rich or lean in oxygen density because the specific substances in the unbalanced exhaust gas are purified to a certain degree by the catalytic converter. However, there is a problem that if the output of the first oxygen sensor is compensated in accordance with the output of the second oxygen sensor, a control frequency of the air fuel ratio control decreases, so that controlability decreases because the output of the first oxygen sensor deviates from that in the normal condition. Further, the purification ratio of the catalytic converter decreases as shown in FIG. 4 showing purification ratio-control frequency characteristic.
Moreover, the catalytic converter deteriorates with years for which it has been used, so that it would not function sufficiently. If such problem occurs, the characteristic of the second oxygen sensor provided downstream from the catalytic converter varies, so that the air fuel ratio of the internal combustion engine cannot not be controlled to improve the purification of the catalytic converter in accordance with the output of the second oxygen sensor.